The production of hydrocarbons from low mobility reservoirs presents significant challenges. Low mobility reservoirs are characterized by high viscosity hydrocarbons, low permeability formations, and/or low driving forces. Any of these factors can considerably complicate hydrocarbon recovery. Extraction of high viscosity hydrocarbons is typically difficult due to the relative immobility of the high viscosity hydrocarbons. For example, some heavy crude oils, such as bitumen, are highly viscous and therefore immobile at the initial viscosity of the oil at reservoir temperature and pressure. Indeed, such heavy oils may be quite thick and have a consistency similar to that of peanut butter or heavy tars, making their extraction from reservoirs especially challenging.
Conventional approaches to recovering such heavy oils often focus on methods for lowering the viscosity of the heavy oil so that the heavy oil may be produced from the reservoir, such as heating the reservoir to lower the viscosity of the heavy oil. Commonly used thermal recovery techniques include a number of reservoir heating methods, such as steam flooding, cyclic steam stimulation, and Steam Assisted Gravity Drainage (SAGD).
Further complicating recovery of hydrocarbons from low mobility reservoirs are hydrocarbons situated below a permafrost zone. Permafrost is a layer of earth that is continuously at or below the freezing point of water, usually for two or more years. Conventional thermal heavy oil recovery techniques often suffer from the disadvantage that the heat from such processes heat and thaw the permafrost. Heating the permafrost is generally undesirable due to its adverse environmental impact on the permafrost. Moreover, thawing the permafrost is known to cause ground subsidence, which adversely and undesirably compromises structures built on top of the permafrost. Indeed, thawing the permafrost often causes catastrophic loss of capital equipment as any structures built upon the permafrost will subside into the thawed permafrost. Structures and capital equipment lost in the permafrost often become permanently irretrievable as no practical methods exist for recovering items lost in thawed permafrost.
These energy-intensive thermal recovery conventional methods are also highly disadvantageous in the particularly colder permafrost regions (e.g. especially high or low latitude geographic regions) due to the high heat loss that necessarily occurs in these colder regions. This larger temperature differential contributes to a more inefficient process due to the higher heat losses. Indeed, In some cases, these thermal recovery techniques are so inefficient that they are often not economically viable for recovering heavy crude oil.
To generate the heat required by conventional thermal technologies, these conventional methods typically use combustion devices to produce the required heat. Unfortunately, these combustion devices produce substantial amounts of greenhouse gases, which are often vented to atmosphere. The accumulation of greenhouse gases such as carbon dioxide in the atmosphere is known to contribute to global warming due to the greenhouse effect. Reducing greenhouse gases in the atmosphere remains a continuing global concern. Despite efforts at reducing carbon dioxide emissions, carbon dioxide concentrations in the atmosphere continue to rise annually primarily due to fossil fuel combustion. The United States Environmental Protection Agency (EPA) estimates that the global atmospheric concentrations of carbon dioxide were 35% higher in 2005 than they were before the Industrial Revolution. Accordingly, these energy-intensive conventional methods suffer from excessive greenhouse gas emissions.
One thermal recovery method involves the use of direct steam generators to generate the heat for enhancing the recovery of the heavy oil. Direct steam generators generate steam by directly injecting water along with the fuel and oxidant to be combusted to produce a single output stream of steam and exhaust gases combined together. Thus, due to the design of direct steam generators, the steam produced necessarily includes the combustion exhaust gases.
Direct steam generators suffer from the disadvantage that they operate at low to moderate pressure. Because of limited experience with these combustion systems at higher pressures, direct steam generators are typically constrained to operate at both low to moderate steam output pressures. This lower pressure design constraint is disadvantageous from the standpoint that some deeper reservoirs require higher pressure steam, which direct steam generators are unable to provide. Another disadvantage of high pressure direct steam generators is that they require significant compression of the fuel and oxidant streams.
Direct steam generators also suffer from the inability to independently control the amount of exhaust gas components that are combined with the steam. Due to the design of direct steam generators, any steam produced will necessarily include all of the exhaust gases combined with the steam. This forced combination of other gases with the steam may be disadvantageous where it is desired to inject steam into a subterranean formation without one or more components of the exhaust gas.
Where air is used as the oxidant to the direct steam generator, the exhaust gas will necessarily contain significant amounts of nitrogen. The inability to feasibly separate the exhaust gas from the steam is also particularly problematic in nitrogen-laden steam where reservoirs are negatively impacted by nitrogen.
Accordingly, there is a need for enhanced heavy oil recovery methods that address one or more of the disadvantages of the prior art.